Such circuit arrangements are known from WO 02/30162 A2, from WO 03/024161 A1 and from US 2002/0041165 A1. All these printed documents address the problems arising with high-pressure discharge lamps, specifically the occurrence of different color characteristics in vertical and horizontal operation. In vertical operation, in particular, color segregation occurs. The causes of this reside in the fact that there is an incomplete mixing of the metal additives in the discharge region. It is proposed as counter measure to excite the second longitudinal acoustic resonance. A fundamental circuit configuration for this purpose is known from the U.S. application bearing the Ser. No. 09/335,020 dated 17 Jun. 1999. However, although the approach described therein does seem suitable for generating the desired drive signal in a laboratory environment, this circuit arrangement is unsuitable for an environment in which appropriate signal generators are not to hand. The solution in accordance with WO 02/30162 is to be described below briefly with reference to the attached FIGS. 1 and 2a. 
The circuit arrangement 10 shown in FIG. 1 firstly comprises a preconditioner 12 that serves the purpose of power factor correction, in particular. Following thereupon is a full-bridge arrangement 14 with four switches. This serves as a commutator and changes the polarity of the voltage signal that is fed to the lamp 16 via the filter circuit 18. A drive circuit 20 that is fed the lamp current IL and the lamp voltage UL as input signal provides as output signal drive signals z(t) that are fed to driver circuits 22 for the switches of the full bridge 14. The drive circuit (20) comprises in general a power control element that operates a signal generator for controlling a PWM (Pulse Width Modulation) module. It can be implemented both in analog fashion and digitally with the aid of a microprocessor. The last named implementation variant is described below by way of example: arranged within the drive circuit 20 is a microprocessor 24, a signal generator 26 and a PWM module 28. With reference to FIG. 2a, in the prior art the PWM module 28 is on the one hand fed at the modulation input a signalx(t)=B0·(1+Â sin fat)·sin f1t that is generated in the signal generator 26 in response to the parameters provided by the microprocessor 24. In this case, the frequency fa of the amplitude modulation is in a range of between 20 and 30 kHz, while the carrier frequency ft is typically swept between 45 and 55 kHz. Consequently, the PWM module 28 must be fed a very complex signal via the signal x(t), in particular the complete signature of the desired signal. The signal y(t) fed via the system input of the PWM module 28 is a function of a constant system frequency f0 (corresponding to a period of T0) that is at 500 kHz in accordance with the prior art: see FIG. 5 of WO 02/30162 in this regard, for example. The following disadvantages, which are significant for practical operation, follow therefrom:
The entire full bridge and drive circuit 14, 20 must be designed for high switching frequencies. This results, on the one hand, in high costs for the required components, and secondly in high switching losses. On the other hand, high demands are placed on the signal generator 26, which must provide the complete signal signature for the PWM module 28 (comparable to the classic class D principle).
As may be gathered from WO 02/30162, page 7, lines 31 to 32, the voltage signal x(t) provided to the PWM module 28 is a low-voltage version of the waveform that is desired for driving the lamp 16. In other words, the entire information is present in the signal x(t).
To sum up, the consequence of this is that although certain advances have been made by comparison with the original approach, mentioned above, in accordance with the U.S. Ser. No. 09/335,020, these are not sufficient to permit the use as a mass produced product at an acceptable price.